Data transmission device, data transmission system, and method

ABSTRACT

A data transmission device ( 1 ) includes a controller ( 2 ), a reception section ( 5 ), a transmission section ( 6 ), and an MPU ( 3 ). The reception section ( 5 ) receives an electric signal sent from a preceding device, and outputs its data to the controller ( 2 ). The transmission section ( 6 ) converts a result of a process by the controller ( 2 ) into an electric signal and transmits it to a successive device. The MPU ( 3 ) controls operation of the controller ( 2 ), the reception section ( 5 ), and the transmission section ( 6 ) in accordance with the operation mode of the device. The reception section ( 5 ) detects cessation of the electric signal sent from the preceding device and, in response to the detection, stops operating. In response to the detection, the transmission section ( 6 ) stops operating and stops sending the electric signal to the successive device.

TECHNICAL FIELD

The present invention relates to a data transmission device, a datatransmission system, and a method therefor, and more specifically todata transmission devices, a data transmission system, and a methodtherefor in which the data transmission devices are connected via atransmission line(s) so as to form a ring structure and in which theyelectrically communicate with one another in a single direction.

BACKGROUND ART

In recent years, in car navigation or when doing the Internet, e.g., ITS(Intelligent Transport Systems), or transmitting image information inspace such as the inside of a motor vehicle, high-volume and high-speedcommunication is required. A great deal of study is being made oncommunication systems for transmitting such digitized video or audiodata, or digital data such as computer data or the like. Also,introduction of a network for transmitting digital data into even spacesuch as the inside of a motor vehicle is becoming more and morewidespread. For example, this intra-vehicle network uses a ring topologyas its physical topology, and connects a plurality of nodes with thering topology to form a unidirectional ring LAN, thus aiming to achieveintegrated connection of an audio device, a navigation device, aninformation terminal device, or the like. For example, Media OrientedSystems Transport (hereinafter referred to as MOST) is used for theaforementioned ring LAN as an information communications protocol. TheMOST refers to not only the communications protocol but also a methodfor constructing a distributed system. Data on a MOST network istransmitted with a frame being a basic unit, and frames are sequentiallytransmitted between the nodes in a single direction.

Noticeably, in the case of a ring LAN provided in the inside of avehicle or the like, radiated noise may cause malfunction of anotherelectronic device disposed on a motor vehicle or the like; besides,there is a necessity to perform accurate transmission without receivingany influence of radiated noise from another device. For this reason, aring LAN using a conventional MOST presupposes that the communicationsprotocol in the MOST is optical communication, and each node isconnected by use of an optical-fiber cable, whereby protection fromnoise is improved while preventing generation of electromagnetic waves.Mean while, in a data transmission system disclosed in InternationalPublication Pamphlet No. 02/30079, data communication is performed withelectric signals using inexpensive cables such as twisted-pair cables orcoaxial cables, while data transmission at high speed exceeding 20 Mbpsis realized with a little radiated noise and improved protection fromnoise.

With reference to FIG. 7, a data transmission system using a ringnetwork in which each node is connected with an inexpensive cable suchas a twisted-pair cable or a coaxial cable is described. FIG. 7 is ablock diagram showing a configuration of the ring network.

In FIG. 7, the ring network is composed of n data transmission devices100 a to 100 n in which each node performs data transmission andreception. To the data transmission devices are connectedconnected-devices 110 a to 110 n each performing a process based on datatransmitted by the data transmission device and outputting its resultantto the data transmission device. Note that as a common hardwareconfiguration, the data transmission devices 100 a to 100 n and theconnected-devices 110 a to 110 n respectively have integral structures.The data transmission devices 100 a to 100 n are connected viatransmission lines 130 a to 130 n composed of coaxial cables ortwisted-pair cables so as to form a ring structure. Each of the datatransmission devices 100 a to 100 n has the same structure, having aprocessing section for processing the communications protocol of thering network, a transmission section, and a reception section (which arenot shown). For example, the transmission section provided in the datatransmission device 100 a outputs data to the reception section providedin the data transmission device 100 b via the transmission line 130 a;and the reception section provided in the data transmission device 100 areceives data from the transmission section provided in the datatransmission device 100 n via the transmission line 130 n.

A data transmission method in which the devices 100 a to 100 n conductoutput to the transmission lines 130 a to 130 n is described. A digitaldata sequence from the connected-device or the like connected to eachone of the data transmission devices 100 a to 100 n is divided by therespective transmission section into units of a predetermined number ofbits to obtain data symbols, which are converted through mapping by useof a conversion table and a filtering process into an analog signal,which in turn is outputted to a corresponding one of the transmissionlines 130 a to 130 n. The analog signal is outputted as a waveform inwhich mapped signal levels are in a predetermined cycle. Then, thereception section of each of the data transmission devices 100 a to 100n receives the analog signal, which is decoded through a filteringprocess and inverse mapping into data symbols, which in turn areconverted into a digital data sequence.

Here, in the case of an intra-vehicle network, while the network is notused, it is required that a mode (hereinafter referred to as a“zero-power mode”) be available in which operation is suspended byturning off main hardware constituting the network in order to reducepower consumption to the least possible. In the case wheretransmission/reception involves conversion into an analog signal asdescribed above, turning off the transmission section and the receptionsection makes it difficult for all data transmission devices to returnfrom the zero-power mode in coordination with one another.

Another conceivable method is to allow the powers of the transmissionsection and reception section of the data transmission device to remainturned on while shifting the processing section and the connected-deviceto the zero-power mode. In this case, however, power consumption of theentire network is large in the zero-power mode. For example, considerthe case where the ring network is provided inside a motor vehicle andoperation of the entire network is to be suspended by shifting the ringnetwork to the zero-power mode when the key of the motor vehicle isturned off. In this case, it is necessary to make power consumption aslittle as possible (to make it nearly zero) because while the key of themotor vehicle is off, there is no electric power generation by anengine, resulting in limited power capacity. In the zero-power mode asdescribed above where the powers of the transmission section andreception section of the data transmission device remain turned on, itis difficult to restrict power consumption to zero. In other words, thezero-power mode where the powers of only the processing section of thedata transmission device and the connected-device are turned off doesnot achieve an essential object of the zero-power mode.

Therefore, an object of the present invention is to provide a datatransmission device, a data transmission system, and a method therefor,which, in a mode involving turning off main hardware constituting a ringnetwork, allows power consumption in that mode to be low and in which itis easy to return to a normal operation mode.

DISCLOSURE OF THE INVENTION

The present invention has the following features to attain the objectabove.

A data transmission device according to the present invention isconnected to a ring-type data transmission network, and electricallycommunicates with another device via a transmission line in aunidirectional manner. The data transmission device includes: aprocessing section for processing received data and data to betransmitted based on a predetermined communications protocol; areception section for receiving an electric signal sent from a precedingdevice and outputting data contained in the electric signal to theprocessing section; a transmission section for converting a result of aprocess by the processing section into an electric signal andtransmitting the electric signal to a successive device; and a controlsection for controlling operation of the processing section, thereception section, and the transmission section in accordance with anoperation mode of its own device. The reception section detectscessation of the electric signal sent from the preceding device and, inresponse to the detection, stops operating. In response to thedetection, the transmission section stops operating and stops sendingthe electric signal to the successive device.

According to the above-described structure of the present invention, ina zero-power mode that stops operation of main hardware, the operationof the reception section and the transmission section included in thedata transmission device is stopped; therefore power consumption of eachof them is reduced, whereby power consumption of the entire device isgreatly reduced. In addition, the data transmission device detects thecessation of the electric signal sent from the preceding datatransmission device, then shifts itself to the zero-power mode, andstops sending the electric signal to the successive data transmissiondevice; therefore, the data transmission devices connected to thering-type data transmission network are able to shift to the zero-powermode in combination.

As a first example, if the cessation of the electric signal sent fromthe preceding device is detected, the reception section transmits, tothe control section, a data cessation signal for indicating thecessation; and based on the data cessation signal transmitted from thereception section, the control section stops operation of the processingsection. As a second example, if the cessation of the electric signalsent from the preceding device is detected, the reception sectiontransmits, to the control section, a data cessation signal forindicating the cessation; based on the data cessation signal transmittedfrom the reception section, the control section outputs a signal forstopping operation of the reception section and the transmissionsection; in response to the signal outputted from the control section inresponse to the detection, the reception section stops operating; and inresponse to the signal outputted from the control section in response tothe detection, the transmission section stops operating and stopssending the electric signal to the successive device. As a thirdexample, a power supply section for supplying power to the processingsection, the reception section, and the transmission section is furtherincluded; if the cessation of the electric signal sent from thepreceding device is detected, the reception section transmits, to thecontrol section, a data cessation signal for indicating the cessation;and based on the data cessation signal transmitted from the receptionsection, the control section stops the power supply section fromsupplying power to the processing section, the reception section, andthe transmission section. By these examples, in the zero-power mode, theoperation of the processing section included in the data transmissiondevice is stopped, or power supply to the transmission section and thereception section is stopped. Therefore, power consumption is furtherreduced, and even in the case where the reception section and thetransmission section do not have functions for turning off the power bythemselves, their operation can be stopped and moreover, powerconsumption of each of them can be completely restricted to zero.

Further, there may be included a signal monitoring section for detectingthe electric signal sent from the preceding device and transmitting, tothe control section, an electric-signal detection signal for indicatingthe detection. In this case, if suspended sending of the electric signalsent from the preceding device is resumed, the signal monitoring sectiondetects the electric signal sent from the preceding device, andtransmits, to the control section, the electric-signal detection signalfor indicating the detection; based on the electric-signal detectionsignal transmitted from the signal monitoring section, the controlsection starts operation of the processing section, the receptionsection, and the transmission section; and by control of the controlsection, the transmission section starts operating and starts sendingthe electric signal to the successive device. Because of this, once thesending of the electric signal from the preceding data transmissiondevice is resumed, the data transmission device which has shifted to thezero-power mode detects the electric signal with the signal monitoringsection, and starts operation of the processing section, the receptionsection, and the transmission section, thereby returning to a normaloperation mode. Therefore, the data transmission device is capable ofallowing the processing section, the reception section, and thetransmission section, which had stopped their operation, to easily starttheir operation, there by returning to the normal operation mode. Inaddition, after returning to the normal operation mode, the datatransmission device resumes sending the electric signal to thesuccessive data transmission device. Therefore, the data transmissiondevices connected to the ring-type data transmission network are able toreturn to the normal operation mode in combination.

For example, the electric signal which the transmission section sends tothe successive device after starting operating by control of the controlsection is a lock signal for establishing clock synchronization. Becauseof this, since the electric signal for allowing the data transmissiondevice to return to the normal operation mode is a lock signal forestablishing clock synchronization, it is possible to simultaneouslyperform a clock reproduction process together with the return process.In addition, for example, the communications protocol used by theprocessing section is defined by Media Oriented Systems Transport(MOST). Because of this, in the case where the data transmission devicesconnected to the ring-type data transmission network perform datacommunication with an electric signal using the MOST as thecommunications protocol, the operation of the reception section and thetransmission section included in the data transmission device is stoppedin the zero-power mode; therefore, power consumption of each of them isreduced, whereby power consumption of the entire device is greatlyreduced. Moreover, the data transmission device detects the cessation ofthe electric signal sent from the preceding data transmission device,shifts itself to the zero-power mode, and stops the electric signal sentto the successive data transmission device; therefore, the datatransmission devices connected to the ring-type data transmissionnetwork are able to shift to the zero-power mode in combination.

A data transmission system according to the present invention includes aplurality of data transmission devices connected via a transmission lineso as to form a ring structure, in which the data transmission deviceselectrically communicate with one another in a unidirectional manner.The data transmission devices each include: a processing section forprocessing received data and data to be transmitted based on apredetermined communications protocol; a reception section for receivingan electric signal sent from a preceding data transmission device andoutputting data contained in the electric signal to the processingsection; a transmission section for converting a result of a process bythe processing section into an electric signal and transmitting theelectric signal to a successive data transmission device; and a controlsection for controlling operation of the processing section, thereception section, and the transmission section in accordance with anoperation mode of its own device, wherein, in at least one of the datatransmission devices, the control section stops operation of theprocessing section, the reception section, and the transmission sectionof its own device based on a predetermined condition for shift, and thetransmission section stops transmission of the electric signal, and inanother data transmission device, the reception section of its owndevice detects cessation of the electric signal sent from a precedingdata transmission device and, in response to the detection, stopsoperating; and the transmission section of its own device stopsoperating in response to the detection and stops sending the electricsignal to a successive data transmission device.

According to the above-described structure of the present invention, inthe zero-power mode which stops operation of main hardware included inthe data transmission device, the operation of the reception section andthe transmission section is stopped; therefore, power consumption ofeach of them is reduced, and power consumption of the entire datatransmission system is greatly reduced. In addition, at least one of thedata transmission devices shifts to the zero-power mode based on thepredetermined condition for shift and thereafter stops sending of theelectric signal from itself; and another data transmission devicedetects the cessation of the electric signal sent from the precedingdata transmission device, shifts itself to the zero-power mode, andstops sending the electric signal to the successive data transmissiondevice. Therefore, the data transmission devices connected to the datatransmission system are able to shift to the zero-power mode incombination.

As a first example, in the other data transmission device, if thecessation of the electric signal sent from the preceding datatransmission device is detected, the reception section transmits, to thecontrol section of its own device, a data cessation signal forindicating the cessation; and based on the data cessation signaltransmitted from the reception section of its own device, the controlsection stops operation of the processing section of its own device. Asa second example, in the other data transmission device, if thecessation of the electric signal sent from the preceding datatransmission device is detected, the reception section transmits, to thecontrol section of its own device, a data cessation signal forindicating the cessation; based on the data cessation signal transmittedfrom the reception section of its own device, the control sectionoutputs a signal for stopping operation of the reception section and thetransmission section of its own device; in response to the signaloutputted from the control section of its own device in response to thedetection, the reception section stops operating; and in response to thesignal outputted from the control section of its own device in responseto the detection, the transmission section stops operating and stopssending the electric signal to the successive data transmission device.As a third example, the data transmission devices each further include apower supply section for supplying power to the processing section, thereception section, and the transmission section of its own device; ifthe cessation of the electric signal sent from the preceding datatransmission device is detected, the reception section transmits, to thecontrol section of its own device, a data cessation signal forindicating the cessation; and based on the data cessation signaltransmitted from the reception section of its own device, the controlsection stops the power supply section of its own device from supplyingpower to the processing section, the reception section, and thetransmission section.

In addition, the data transmission devices may each further include asignal monitoring section for detecting the electric signal sent fromthe preceding data transmission device and transmitting, to the controlsection, an electric-signal detection signal for indicating thedetection. In this case, in at least one of the data transmissiondevices, based on a predetermined return condition, the control sectionstarts operation of the processing section, the reception section, andthe transmission section of its own device in stopped state, and thetransmission section resumes the transmission of the electric signal,and in another data transmission device, if suspended sending of theelectric signal sent from the preceding data transmission device isresumed, the signal monitoring section detects the electric signal sentfrom the preceding data transmission device, and transmits, to thecontrol section of its own device, the electric-signal detection signalfor indicating the detection; based on the electric-signal detectionsignal transmitted from the signal monitoring section, the controlsection starts operation of the processing section, the receptionsection, and the transmission section of its own device; and thetransmission section starts operating and starts sending the electricsignal to the successive data transmission device. Because of this, inthe data transmission system which has shifted to the zero-power mode,at least one of the data transmission devices returns to a normaloperation mode based on the predetermined return condition andthereafter resumes the sending of the electric signal from itself; andonce the sending of the electric signal from the preceding datatransmission device is resumed, another data transmission device detectsthe electric signal with the signal monitoring section, and startsoperation of the processing section, the reception section, and thetransmission section, thereby returning to the normal operation mode.Therefore, the data transmission device is capable of allowing theprocessing section, the reception section, and the transmission section,which had stopped their operation, to easily start their operation,thereby returning to the normal operation mode. In addition, afterreturning to the normal operation mode, each data transmission deviceresumes sending the electric signal to the successive data transmissiondevice. Therefore, the data transmission devices connected to the datatransmission network are able to return to the normal operation mode incombination.

For example, the electric signal which each transmission section sendsto the successive data transmission device after starting operating bycontrol of the control section is a lock signal for establishing clocksynchronization with each other. In addition, the data transmissiondevice which resumes the transmission of the electric signal based onthe predetermined return condition is, for example, a master, whichperforms data transmission with a clock held by itself and is connectedto the data transmission system. Further, the communications protocolused by the processing section is defined by MOST, for example.

In a data transmission method according to the present invention, aplurality of nodes are connected via a transmission line so as to form aring structure, and each node electrically communicates with one anotherin a unidirectional manner. The data transmission method includes: aprocessing step, performed by each node, of processing received data anddata to be transmitted based on a predetermined communications protocol;a reception step, performed by each node, of receiving an electricsignal sent from a preceding node and sending data contained in theelectric signal to the processing step; a transmission step, performedby each node, of transmitting a result of a process by the processingstep to a successive node as an electric signal; and a control step,performed by each node, of controlling operation of the processing step,the reception step, and the transmission step in accordance with anoperation mode, wherein, in at least one of the nodes, the control stepstops operation by the processing step, the reception step, and thetransmission step of the node based on a predetermined condition forshift, and the transmission step stops transmission of the electricsignal, and in another node, the reception step detects cessation of theelectric signal sent from a preceding node and, in response to thedetection, stops operation; and the transmission step of its own nodestops operation in response to the detection and stops sending theelectric signal to a successive node.

According to the above-described structure of the present invention, inthe zero-power mode which stops operation of main hardware included ineach node, operation by the reception step and the transmission step isstopped; therefore, power consumption required for their operation isreduced, and power consumption of each entire node connected so as toform a ring structure is greatly reduced. In addition, at least one ofthe nodes shifts to the zero-power mode based on the predeterminedcondition for shift and thereafter stops the sending of the electricsignal from itself; and another node detects the cessation of theelectric signal sent from the preceding node, shifts itself to thezero-power mode, and stops sending the electric signal to the successivenode. Therefore, each node connected so as to form a ring structure isable to shift to the zero-power mode in combination.

As a first example, in the other node, if the cessation of the electricsignal sent from the preceding node is detected, the reception stepsends, to the control step of its own node, a notification indicatingthe cessation, and based on the notification sent by the reception stepof its own node, the control step stops operation by the processing stepof its own node. As a second example, in the other node, if thecessation of the electric signal sent from the preceding node isdetected, the reception step sends, to the control step of its own node,a notification indicating the cessation; based on the notification sentby the reception step of its own node, the control step sends anotification for stopping operation by the reception step and thetransmission step of its own node; in response to the notification sentby the control step of its own node in response to the detection, thereception step stops operation, and in response to the notification sentby the control step of its own node in response to the detection, thetransmission step stops operation and stops sending the electric signalto the successive node. As a third example, the nodes each furtherinclude a power supply step of supplying power used for operation in theprocessing step, the reception step, and the transmission step; if thecessation of the electric signal sent from the preceding node isdetected, the reception step sends, to the control step of its own node,a notification indicating the cessation; and based on the notificationsent by the reception step of its own node, the control step stops thepower supply step of its own node from supplying power used foroperation of the processing step, the reception step, and thetransmission step.

In addition, the nodes may each further include a signal monitoring stepof detecting the electric signal sent from the preceding node andsending, to the control step, a notification indicating the detection.In this case, in at least one of the nodes, based on a predeterminedreturn condition, the control step starts operation by the processingstep, the reception step, and the transmission step of its own node instopped state, and the transmission step resumes the transmission of theelectric signal, and in another node, if suspended sending of theelectric signal sent from the preceding node is resumed, the signalmonitoring step detects the electric signal sent from the precedingnode, and sends, to the control step of its own node, the notificationindicating the detection; based on the notification indicating thedetection sent by the signal monitoring step, the control step startsoperation by the processing step, the reception step, and thetransmission step of its own node; and operation by the transmissionstep is started to start the sending of the electric signal to thesuccessive node. Because of this, regarding the nodes which have shiftedto the zero-power mode, at least one of the nodes returns to the normaloperation mode based on the predetermined return condition andthereafter resumes the sending of the electric signal by itself; andonce the sending of the electric signal from the preceding node isresumed, another node detects the electric signal with the signalmonitoring step and starts operation by the processing step, thereception step, and the transmission step, thereby returning to thenormal operation mode. Therefore, the node is capable of allowing theprocessing step, the reception step, and the transmission step, whichhad stopped their operation, to easily start their operation, there byreturning to the normal operation mode. In addition, after returning tothe normal operation mode, each node resumes sending the electric signalto the successive node. Therefore, the nodes connected so as to form aring structure are able to return to the normal operation mode incombination.

For example, the electric signal which each transmission step sends tothe successive node after starting operation by control of the controlstep is a lock signal for establishing clock synchronization with eachother. The node which resumes the transmission of the electric signalbased on the predetermined return condition may be a master, whichperforms data transmission with a clock held by itself. In addition, thecommunications protocol used by the processing step is defined by MOST,for example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of a data transmissionsystem according to an embodiment of the present invention.

FIG. 2 is a functional block diagram showing a structure of a datatransmission device 1 in FIG. 1.

FIG. 3 is a flowchart showing an operation of the data transmissionsystem of FIG. 1 for shifting from a normal operation mode to azero-power mode.

FIG. 4 is a flowchart showing an operation of the data transmissionsystem of FIG. 1 for returning from the zero-power mode to the normaloperation mode.

FIG. 5 is a flowchart showing another exemplary operation of the datatransmission system of FIG. 1 for shifting from the normal operationmode to the zero-power mode.

FIG. 6 is a flowchart showing another exemplary operation of the datatransmission system of FIG. 1 for returning from the zero-power mode tothe normal operation mode.

FIG. 7 is a block diagram showing a configuration of a conventional ringnetwork.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to FIG. 1, a data transmission system according to anembodiment of the present invention is described. FIG. 1 is a blockdiagram showing a configuration of the data transmission system.

In FIG. 1, the data transmission system has a ring topology as itsphysical topology in which a plurality of nodes are connected accordingto the ring topology, thereby forming a unidirectional ring LAN. Anexample of such a data transmission system is described below where thenodes are composed of six data transmission devices 1 a to 1 f, whichare connected via transmission lines 80 a to 80 f so as to form a ringstructure, and transmitted data is transmitted via the transmissionlines 80 a to 80 f in a single direction. To the data transmissiondevices 1 a to 1 f are connected connected-devices (e.g., audio devices,navigation devices, or information terminal devices) 10 a to 10 f eachconducting a process based on data transmitted through the datatransmission system and outputting its resultant to the datatransmission system. Note that as a common hardware configuration, thedata transmission devices 1 a to 1 f and the connected-devices 10 a to10 f respectively have integral structures.

For example, Media Oriented Systems Transport (hereinafter referred toas MOST) is used as an information communications protocol for theabove-described data transmission system. Data transmitted using theMOST as the communications protocol is transmitted with a frame being abasic unit, and frames are sequentially transmitted between the datatransmission devices 1 in a single direction. In other words, the datatransmission device 1 a outputs data to the data transmission device 1 bvia the transmission line 80 a. The data transmission device 1 b outputsthe data to the data transmission device 1 c via the transmission line80 b. The data transmission device 1 c outputs the data to the datatransmission device 1 d via the transmission line 80 c. The datatransmission device 1 d outputs the data to the data transmission device1 e via the transmission line 80 d. The data transmission device 1 eoutputs the data to the data transmission device 1 f via thetransmission line 80 e. The data transmission device 1 f outputs thedata to the data transmission device 1 a via the transmission line 80 f.Inexpensive cables such as twisted-pair cables or coaxial cables areused as the transmission lines 80 a to 80 f,and the data transmissiondevices 1 mutually perform data communication with an electric signal.Here, in the data transmission system, the data transmission device 1 ais a master, which transmits data with a clock of its own, and the otherdata transmission devices 1 b to 1 f are slaves, which operatesynchronized with a clock generated at the master.

Next, with reference to FIG. 2, a structure of the data transmissiondevice 1 is described. FIG. 2 is a functional block diagram showing thestructure of the data transmission device 1. Note that theabove-described plurality of data transmission devices 1 a to 1 f eachhave the same structure.

In FIG. 1, the data transmission devices 1 each includes a controller 2,a microcomputer (MPU) 3, a transmission/reception section 4, a powersupply section 8, and an activity detection section 9. For example, thecontroller 2 is composed of an LSI and, in the case where thecommunications protocol used in the data transmission system is theMOST, performs a predetermined data transmission/reception process ofthe MOST. The description below is made using the MOST as an exemplarycommunications protocol used in the data transmission system.

To the controller 2 is connected the connected-device 10 which performsa process based on data transmitted in the data transmission system andoutputs its resultant to the data transmission system. As its function,the controller 2 converts data from the connected-device 10 connectedthereto into a protocol stipulated by the MOST, then outputting digitaldata TX to the transmission/reception section 4, and processes digitaldata RX outputted from the transmission/reception section 4, thentransmitting it to the connected-device 10 connected thereto. Inaddition, the controller 2 outputs, to the transmission/receptionsection 4, an operation mode signal ST indicating a current operationmode.

The MPU 3 controls the controller 2, the transmission/reception section4, and the aforementioned connected-device 10 based on a transmissionmode of the data transmission device 1. For example, the MPU 3 controlsthe data transmission device 1 with respect to a reset function, a powercontrol (a zero-power mode control, described below, on the controller 2and the transmission/reception section 4), a master/slave selectionprocess, a process of shifting to a diag mode, a scramble transmissionfunction, etc.

A clock control section 7 controls a clock of the data transmissiondevice 1: for example, it reproduces a clock generated at another datatransmission device 1, reproduces a clock for the controller 2, andoutputs a clock used in a transmission section 6.

The transmission/reception section 4, typically composed of an LSI,includes a reception section 5, the transmission section 6, and theclock control section 7. The reception section 5 receives an electricsignal Min inputted from a preceding data transmission device 1 via thetransmission line 80, and, based on a clock reproduced at the clockcontrol section 7, converts the electric signal Min into a digitalsignal RX, and then outputs it to the controller 2. In addition, thereception section 5 reproduces a clock component included in theelectric signal Min and outputs it to the clock control section 7. Inaddition, the reception section 5 outputs a reception operation modesignal NST to the MPU 3 based on the presence or absence of input of theelectric signal Min from the preceding data transmission device 1 viathe transmission line 80. Based on the clock of the clock controlsection 7, the transmission section 6 converts the digital data TXoutputted from the controller 2 into an electric signal Mout, thenoutputting it to a successive data transmission device 1 via thetransmission line 80.

A function of the transmission section 6 is described in detail. Thetransmission section 6 has inside it an S/P (serial/parallel) conversionsection, a mapping section, a roll off filter, a DAC (a digital toanalog converter), a differential driver, a training signal generationsection, and the like. For example, by their operation, the transmissionsection 6 converts the digital data TX into an analog electric signalMout subjected to eight-value mapping, then outputting it. The functionof the transmission section 6 is described below with reference to anexemplary case where conversion is made into an analog signal Moutsubjected to eight-value mapping.

First, based on a clock controlled by the clock control section 7, thetransmission section 6 selects data (e.g., digital data TX) to betransmitted to the transmission line 80 and performs an S/P conversionprocess. This S/P conversion process converts serial digital data TXoutputted from the controller 2 into parallel form in order to performmultilevel transmission. In the case where the communications protocolis the MOST, since the output from the controller 2 is made in the formof serial digital data TX, the S/P conversion process converts seriallyinputted data into two-bit parallel data. Regarding the clock, by theclock control section 7, used by the transmission section 6: in the casewhere the data transmission device 1 is the master, a clock reproducedby a transmission-side PLL (Phase Locked Loop) based on a clock held byitself is used; whereas in the case where it is a slave, a clockcomponent of a signal received from the transmission line 80 isextracted, and a clock reproduced by a reception-side PLL is used as asystem clock. The transmission-side PLL and the reception-side PLL areboth contained in the clock control section 7.

Next, the transmission section 6 maps the two-bit parallel datasubjected to the S/P conversion and a training signal TS outputted fromthe training signal generation section onto one of eight values ofsymbols based on the aforementioned system clock. In this mapping, inorder that another data transmission device 1 disposed on the receivingside may perform clock reproduction, the two-bit parallel data isallocated alternately to upper four values of symbols and lower fourvalues of symbols among the eight values of symbols. In addition, inorder to exclude influence of fluctuation or difference ofdirect-current components between transmission and reception, themapping is performed based on difference from a previous value.Moreover, regarding the signal after mapping, the transmission section 6limits the bandwidth of the electric signal which is to be transmittedand controls intersymbol interference by using a roll off filter. Forthis roll off filter, which is, for example, a wave-shaping filter, a33-tap, 12-bit FIR filter characterized by performing route distributionon a roll-off rate of 100% with a sampling frequency four times a symbolrate is used, for example.

Next, by using the DAC, the transmission section 6 converts into ananalog signal the signal whose bandwidth has been limited by the rolloff filter. Then, using the differential driver, the transmissionsection 6 amplifies the intensity of the analog signal outputted fromthe DAC and converts it into a differential signal, then sending it tothe transmission line 80. For a pair of lead wires included in thetransmission line 80, the differential driver transmits the electricsignal, which is sent, to one side (a positive side) of the lead wiresin the transmission line 80, while transmitting a signal whose positiveand negative are inverse to those of the electric signal to the otherside (a negative side) in the transmission line 80. Thus, the electricsignals of the positive side and the negative side are sent, as a pair,to the transmission line 80, whereby influence of noise radiated fromthe transmission line 80 and common mode noise introduced from outsidecan be reduced.

The training signal generation section in the transmission section 6generates a predetermined training signal TS for setting determinationlevels, which serve as a criterion for data determination made inassociation with the other data transmission device 1 disposed on thereceiving side. Similarly to the above-described digital data TX, thetraining signal TS generated by the training signal generation sectionis subjected to mapping, analog conversion, and the like, and then sentto the transmission line 80.

Next, a function of the reception section 5 is described in detail. Thereception section 5 has a differential receiver, an ADC (analog todigital converter), a roll off filter, a inverse mapping section, a P/S(Parallel/Serial) conversion section, a clock reproduction section, andthe like.

First, by using the differential receiver, the reception section 5converts an electric signal Min inputted from the transmission line 80into a voltage signal. As described above, transmission is performedwith a differential signal, which is composed of the positive side andthe negative side paired for the pair of lead wires contained in thetransmission line 80. The differential receiver, which determines asignal based on difference between the positive side and the negativeside, works effectively against influence of common mode noiseintroduced from outside. Then, by using the ADC, the reception section 5converts into a digital signal the voltage signal obtained by theconversion by the differential receiver.

Next, by using the roll off filter, the reception section 5 performsnoise reduction on the digital signal obtained by the conversion by theADC. For this roll off filter, which is also an FIR filter for waveformshaping, an FIR filter with sixteen times a symbol rate is used, forexample. It realizes a roll-off characteristic without intersymbolinterference in conjunction with the above-described roll off filter ofthe transmission section 6. Then, by using the inverse mapping sectionand based on a clock reproduced by the clock reproduction section, thereception section 5 reproduces data before being subjected to mapping bythe mapping section on the transmitting side based on difference betweena received data value and a previous value. A process at the inversemapping section is performed, using as a criterion the determinationlevels set by the training signal TS as described above, and thedetermination levels are used as ideal values in difference. By thisinverse mapping process, a received signal is converted into paralleldata. Then, the reception section 5 performs a P/S conversion process onthe parallel data obtained after the inverse mapping process to obtainserial digital data RX and outputs it to the controller 2.

The clock reproduction section in the reception section 5 detects theclock component of the signal received from the transmission line 80,which is outputted from the ADC, thereby reproducing a transmission lineclock. The clock reproduced by the clock reproduction section isoutputted to the clock control section 7 to be used as a reference clockfor the reception-side PLL.

Based on power control by the MPU 3, the power supply section 8 suppliespower to the controller 2, the transmission/reception section 4, theactivity detection section 9, etc. The activity detection section 9,which is typically composed of an electric circuit having a comparator,etc., monitors the electric signal Min from the transmission line 80,which is inputted to the data transmission device. In addition, if anelectric signal Min is detected in a zero-power mode described below,the activity detection section 9 reports the detection to the MPU 3.

In the case of a data transmission system inside a vehicle, powercapacity available for the data transmission system is limited, forexample; therefore, while a network is not used, it is necessary toshift to a mode (zero-power mode) in which operation is suspended whilemain hardware constituting the network is turned off to consume aslittle power as possible. With reference to FIGS. 3 and 4, a process, inthe data transmission system, of shifting from a normal operation modeto the zero-power mode and thereafter returning from the zero-power modeto the normal operation mode is described below. FIG. 3 is a flowchartshowing an operation of the data transmission system for shifting fromthe normal operation mode to the zero-power mode, and FIG. 4 is aflowchart showing an operation of the data transmission system forreturning from the zero-power mode to the normal operation mode.

First, with reference to FIG. 3, the operation of the data transmissionsystem for shifting from the normal operation mode to the zero-powermode is described. The shifting operation of the data transmissiondevice described below is applicable to any system in which a pluralityof data transmission devices 1 are connected so as to form a ringstructure. However, for making a specific explanation, the descriptionis made with reference to an exemplary case where six data transmissiondevices 1 a to 1 f are connected via transmission lines 80 a to 80 f soas to form a ring structure (see FIG. 1). Note that as described above,in the data transmission system, the data transmission device 1 a is themaster which transmits data with the clock of its own, while the otherdata transmission devices 1 b to 1 f are slaves which operatesynchronized with a clock generated at the master.

In FIG. 3, all data transmission devices 1 a to 1 f connected to thedata transmission system are in the normal operation, exchanging databetween one another (steps S11 and S51). The master data transmissiondevice 1 a determines, during the normal operation, whether or not toshift to the zero-power mode (step S12), and, if the shift to thezero-power mode is not conducted, continues the above step S11.

The determination instep S12 as to the shift to the zero-power mode istypically performed by the MPU 3 included in the master datatransmission device 1 a.For example, in the case where the datatransmission system is provided inside a motor vehicle, the MPU 3performs the shift to the zero-power mode as a result of a key of themotor vehicle being turned off, or determines the shift to thezero-power mode based on an instruction given by a user operating aswitch. In the case where a condition for the shift to the zero-powermode is previously set in the controller 2 included in the master datatransmission device 1 a, the controller 2 may determine the shift to thezero-power mode based on the condition for the shift.

If, at the above-described step S12, the MPU 3 included in the masterdata transmission device 1 a determines to shift to the zero-power mode,the MPU 3 notifies the controller 2 of its own device to shift to thezero-power mode, and the controller 2 shifts to the zero-power mode(step S13). Next, in order to notify the transmission/reception section4 of its own device to shift to the zero-power mode, the controller 2included in the master data transmission device 1 a changes an operationmode signal ST from Low(0) to High(1) and outputs it to thetransmission/reception section 4, and stops output of the digital dataTX (step S14).

Next, the transmission/reception section 4 included in the master datatransmission device 1 a shifts to the zero-power mode as a result of theoutput of the operation mode signal ST from the controller 2 of its owndevice becoming High(1) and the output of the digital data TX havingbeen stopped (step S15). Then, the transmission/reception section 4stops output of the electric signal Mout being outputted from thetransmission section 6 to the transmission line 80 a (step S16).

Through the processes of the above-described steps S12 to S16, themaster data transmission device 1 a completes the shift to thezero-power mode. This zero-power mode eliminates the need of theoperation of the controller 2 and transmission/reception section 4included in the data transmission device 1 a. Through the processes ofthe above-described steps S12 to S16, the controller 2 and thetransmission/reception section 4 are able to reduce power consumption tothe utmost by their own functions. Note that, however, after the processof the above-described step S16, the MPU 3 may perform power control onthe power supply section 8 to stop supplying power to the controller 2and the transmission/reception section 4. Moreover, if necessary, powersupply to the connected-device 10 a connected to the data transmissiondevice 1 a may also be stopped.

In the above-described data transmission system that shifts from thenormal operation mode to the zero-power mode, the controller 2 outputsthe operation mode signal ST of High(1) to the transmission/receptionsection 4 at step S14, and in response thereto, thetransmission/reception section 4 shifts to the zero-power mode. However,the MPU 3 may directly instruct the transmission/reception section 4 toshift to the zero-power mode by use of an operation mode signal. In thiscase, regarding the master data transmission device 1, if the MPU 3thereof determines to shift to the zero-power mode at theabove-described step S12, the MPU 3 notifies the transmission/receptionsection 4 of its own device to shift to the zero-power mode by use ofthe operation mode signal, and then the transmission/reception section 4shifts to the zero-power mode.

In the case where the MPU 3 directly instructs thetransmission/reception section 4 to shift to the zero-power mode, powersupply to the transmission/reception section 4 maybe stopped in orderfor the transmission/reception section 4 to shift to the zero-powermode. In this case, regarding the master data transmission device 1, ifthe MPU 3 thereof determines to shift to the zero-power mode at theabove-described step S12, the MPU 3 stops power supply from the powersupply section 8 of its own device to the transmission/reception section4, whereby the transmission/reception section 4 shifts to the zero-powermode.

Meanwhile, the slave data transmission devices 1 b to 1 f eachdetermines, in the aforementioned normal operation, the presence orabsence of an input of the electric signal Min from the transmissionline 80 (step S52), and, if there is an input of the electric signalMin, continues the above-described step S51. Then, if the master datatransmission device 1 a performs the above-described step S16, therebystopping the output of the electric signal Mout being outputted to thetransmission line 80 a, the input of the electric signal Min to theslave data transmission device 1 b successively connected thereto viathe transmission line 80 a ceases. If the input of the electric signalMin has ceased, the transmission/reception section 4 included in theslave data transmission device 1 b changes the reception operation modesignal NST from High(1) to Low(0) and outputs it to the MPU 3 of its owndevice (step S53).

Next, in response to the output of the reception operation mode signalNST becoming Low(0), the MPU 3 included in the slave data transmissiondevice 1 b shifts to the zero-power mode (step S54). Then, the MPU 3notifies the controller 2 of its own device to shift to the zero-powermode.

Next, the controller 2 included in the slave data transmission device 1b shifts to the zero-power mode (step S55), and, in order to notify thetransmission/reception section 4 of its own device to shift to thezero-power mode, changes the operation mode signal ST from Low(0) toHigh(2) and outputs it to the transmission/reception section 4, thenstopping the output of the digital data TX (step S56).

Next, as a result of the output of the operation mode signal ST from thecontroller 2 of its own device becoming High(1) and the output of thedigital data TX having been stopped, the transmission/reception section4 included in the slave data transmission device 1 b shifts to thezero-power mode (step S57). Then, the transmission/reception section 4stops the output of the electric signal Mout being outputted from thetransmission section 6 to the transmission line 80 b (step S58).

Through the processes of the above-described steps S52 to S58, the slavedata transmission device 1 b completes the shift to the zero-power mode.Similarly to the master data transmission device 1 a, this zero-powermode eliminates the need of the operation of the controller 2 andtransmission/reception section 4 included in the slave data transmissiondevice 1 b. Through the processes of the above-described steps S52 toS58, the controller 2 and the transmission/reception section 4 are ableto reduce power consumption to the utmost by their own functions. Notethat, however, after the process of the above-described step S58, theMPU 3 may perform power control on the power supply section 8, therebystopping power supply to the controller 2 and the transmission/receptionsection 4. Moreover, if necessary, power supply to the connected-device10 b connected to the data transmission device 1 b may also be stopped.

In the above-described data transmission system that shifts from thenormal operation mode to the zero-power mode, the controller 2 outputsthe operation mode signal ST of High(1) to the transmission/receptionsection 4 at step S56, and in response thereto, thetransmission/reception section 4 shifts to the zero-power mode. However,the MPU 3 may directly instruct the transmission/reception section 4 toshift to the zero-power mode. In this case, regarding the slave datatransmission device 1, if the MPU 3 of its own has shifted to thezero-power mode at the above-described step S54, the MPU 3 notifies thetransmission/reception section 4 of its own device to shift to thezero-power mode, and then the transmission/reception section 4 shifts tothe zero-power mode.

In the case where the MPU 3 directly instructs thetransmission/reception section 4 to shift to the zero-power mode, powersupply to the transmission/reception section 4 maybe stopped in orderfor the transmission/reception section 4 to shift to the zero-powermode. In this case, regarding the slave data transmission device 1, ifthe MPU 3 of its own has shifted to the zero-power mode at theabove-described step S54, the MPU 3 stops power supply from the powersupply section 8 of its own device to the transmission/reception section4, whereby the transmission/reception section 4 shifts to the zero-powermode.

The operation for shifting to the zero-power mode applied to the datatransmission device 1 b also applies to the other slave datatransmission devices 1 c to 1 f. That is, as a result of the input ofthe electric signal Min inputted from the transmission line 80 b havingbeen stopped, the data transmission device 1 c shifts to the zero-powermode; as a result of the input of the electric signal Min inputted fromthe transmission line 80 c having been stopped, the data transmissiondevice 1 d shifts to the zero-power mode; as a result of the input ofthe electric signal Min inputted from the transmission line 80 c havingbeen stopped, the data transmission device 1 e shifts to the zero-powermode; and, as a result of the input of the electric signal Min inputtedfrom the transmission line 80 e having been stopped, the datatransmission device 1 f shifts to the zero-power mode. The combinationof these operations causes all data transmission devices 1 a to 1 fconnected to the data transmission system to shift to the zero-powermode.

Next, with reference to FIG. 4, the operation of the data transmissionsystem for returning from the zero-power mode to the normal operationmode is described. The return operation of the data transmission devicedescribed below is also applicable to any system in-which a plurality ofdata transmission devices 1 are connected so as to form a ringstructure. However, for making a specific explanation, the descriptionis made with reference to an exemplary case where six data transmissiondevices 1 a to 1 f are connected via transmission lines 80 a to 80 f soas to form a ring structure (see FIG. 1). Note that, as described above,when the data transmission system returns, the data transmission device1 a is the master which transmits data with the clock of its own, andthe other data transmission devices 1 b to 1 f are slaves which operatesynchronized with the clock generated at the master.

In FIG. 4, all data transmission devices 1 a to 1 f connected to thedata transmission system are all operating in the zero-power mode (stepsS21 and S61). Then, during the aforementioned zero-power mode, themaster data transmission device 1 a determines whether or not to returnto the normal operation mode (step S22), and, if it does not return tothe normal operation mode, continues the above-described step S21.

The determination in step S22 as to returning to the normal operationmode is typically performed based on a condition for return, which isset in the MPU 3 included in the master data transmission device 1 a.For example, in the case where the data transmission system is providedinside a motor vehicle and has shifted to the zero-power mode as aresult of a key of the motor vehicle being turned off, the MPU 3 maydetermine to return to the normal operation mode as a result of the keyof the motor vehicle being turned on, or may determine to return to thenormal operation mode based on an instruction given by a user operatinga switch.

If the MPU 3 included in the master data transmission device 1 a hasdetermined at the above-described step S22 to return to the normaloperation mode, the MPU 3 activates the controller 2 andtransmission/reception section 4 of its own device (step S23). Regardingthe activation at the above-described step S23, in the case where powersupply to be supplied from the power supply section 8 has been stoppedin order for the controller 2 and the transmission/reception section 4to shift to the zero-power mode, the MPU 3 controls the power supplysection 8 to resume power supply to the controller 2 and thetransmission/reception section 4. Further, in the case where thecontroller 2 and the transmission/reception section 4 have limited powerconsumption to zero by their own functions to shift to the zero-powermode, the MPU 3 performs an activation process by instructing each ofthem to become activated and reset. In this activation process, thecontroller 2 included in the master data transmission device 1 a isoutputting the operation mode signal ST as Low(0) to thetransmission/reception section 4 (step S24).

Next, based on the operation mode signal ST outputted as Low(0) from thecontroller 2 of its own device, the transmission/reception section 4included in the master data transmission device 1 a, which has beenactivated by the above-described step S23, shifts to the normaloperation mode. Then, the transmission/reception section 4 performs aninitialization operation on a physical layer, and, in the initializationoperation, establishes clock synchronization with each data transmissiondevice. Based on an output clock of a transmission PLL controlled by theclock control section 7 of its own device, the transmission/receptionsection 4 transmits, to the transmission line 80 a, a lock signal LS forestablishing clock synchronization with another data transmissiondevice, as the electric signal Mout (step S25). This lock signal LS is,for example, a sinusoidal signal based on a clock frequency of thetransmission PLL included in the master data transmission device 1 a.

Meanwhile, the slave data transmission devices 1 b to 1 f are eachoperating in the zero-power mode, and the activity detection section 9of its own device monitors the presence or absence of input of theelectric signal Min from the transmission line 80 (step S62). If thereis no input of the electric signal Min, the above-described step S61continues. Then, if the master data transmission device 1 a performs theabove-described step S25, thereby outputting the lock signal LS as theelectric signal Mout to the transmission line 80 a, the input of theelectric signal Min to the slave data transmission device 1 bsuccessively connected thereto via the transmission line 80 a isstarted. If any input of the electric signal Min is detected, theactivity detection section 9 included in the slave data transmissiondevice 1 b outputs to the MPU 3 of its own device an activity detectionsignal for indicating that detection (step S63).

If, at step S63, there is any input of the activity detection signalfrom the activity detection section 9 included in the slave datatransmission device 1 b, the MPU 3 of its own device activates thecontroller 2 and transmission/reception section 4 of its own device(step S64). Regarding the activation at the above-described step S64, inthe case where power supply to be supplied from the power supply section8 has been stopped in order for the controller 2 and thetransmission/reception section 4 to shift to the zero-power mode, theMPU 3 controls the power supply section 8 to resume power supply to thecontroller 2 and the transmission/reception section 4. Further, in thecase where the controller 2 and the transmission/reception section 4have limited power consumption to zero by their own functions to shiftto the zero-power mode, the MPU 3 performs an activation process byinstructing each of them to become activated and reset. In thisactivation process, the controller 2 included in the slave datatransmission device 1 b outputs the operation mode signal ST as Low(0)to the transmission/reception section 4 (step S65).

Next, based on the operation mode signal ST outputted as Low(0) from thecontroller 2 of its own device, the transmission/reception section 4included in the slave data transmission device 1 b, which has beenactivated by the above-described step S64, shifts to the normaloperation mode. Then, the transmission/reception section 4 reproduces aclock with the clock reproduction section of its own device (step S66),and transmits a lock signal LS to the transmission line 80 b based onthe clock outputted by its own reception PLL (step S67).

The operation for returning to the normal operation mode applied to thedata transmission device 1 b also applies to the other slave datatransmission devices 1 c to 1 f. That is, as a result of the activitydetection section 9 detecting an input of the electric signal Mininputted from the transmission line 80 b, the data transmission device 1c returns to the normal operation mode; as a result of the activitydetection section 9 detecting an input of the electric signal Mininputted from the transmission line 80 c, the data transmission device 1d returns to the normal operation mode; as a result of the activitydetection section 9 detecting an input of the electric signal Mininputted from the transmission line 80 d, the data transmission device 1e returns to the normal operation mode; and, as a result of the activitydetection section 9 detecting an input of the electric signal Mininputted from the transmission line 80 e, the data transmission device 1f returns to the normal operation mode. That is, as a result of thecombination of these return operations, the return to the zero-powermode is performed in regular order, starting with the master datatransmission device 1 a. Then, the transmission/reception section 4 ofthe data transmission device 1 f reproduces a clock with the clockreproduction section of its own device, and transmits a lock signal LSto the transmission line 80 f based on the clock outputted from thereception PLL.

After transmitting the lock signal LS to the data transmission device 1b at the above-described step S25, the master data transmission device 1a continues to be ready for a reception of the lock signal LStransmitted from the data transmission device 1 f via the transmissionline 80 f (step S26). Then, if the slave data transmission device 1 fperforms the above-described step S67, thereby outputting the locksignal LS as the electric signal Mout to the transmission line 80 f,thetransmission/reception section 4 included in the master datatransmission device 1 a receives the lock signal LS from thetransmission line 80 f and reproduces the clock with the clockreproduction section of its own device. Thus, clock synchronization ofthe entire data transmission system is established.

Thereafter, the data transmission devices 1 a to 1 f transmit andreceive a training signal for setting a criterion for data receptionbetween them, thereby setting a criterion for data determination in thenormal operation mode, and then start data transmission and reception(steps S27 and S68). Then, the process according to this flowchart iscompleted.

Note that in the above-described data transmission system shifting fromthe normal operation mode to the zero-power mode, thetransmission/reception section 4 stops outputting the electric signalMout to the successive data transmission device 1, as a result of thecontroller 2 making notification to the transmission/reception section 4at step S14 or S56. Alternatively, the MPU 3 directly outputs to thetransmission/reception section 4 a notification of the operation modesignal for shifting to the zero-power mode, or stops power supply to thetransmission/reception section 4, so that the transmission/receptionsection 4 stops output of the electric signal Mout to the successivedata transmission device 1. That is, in the above-described datatransmission system shifting from the normal operation mode to thezero-power mode, in response to, for example, a notification by the MPU3 or controller 2 of its own device, the transmission/reception section4 stops output of the electric signal Mout to the successive datatransmission device 1. However, the transmission/reception section 4 maystop output of the electric signal Mout to the successive datatransmission device 1 without any instruction from another component.With reference to FIG. 5 and FIG. 6, the aforementioned process in whichthe transmission/reception section 4 stops output of the electric signalMout to the successive data transmission device 1 is described below.Note that FIG. 5 is a flowchart showing an operation of the datatransmission system for shifting from the normal operation mode to thezero-power mode, and FIG. 6 is a flowchart showing an operation of thedata transmission system for returning from the zero-power mode to thenormal operation mode.

First, with reference to FIG. 5, the operation of the data transmissionsystem for shifting from the normal operation mode to the zero-powermode is described. The shifting operation of the data transmissiondevice described below is applicable to any system in which a pluralityof data transmission devices 1 are connected so as to form a ringstructure. However, for making a specific explanation, the descriptionis made with reference to an exemplary case where six data transmissiondevices 1 a to 1 f are connected via transmission lines 80 a to 80 f soas to form a ring structure (see FIG. 1). Note that as described above,in the data transmission system, the data transmission device 1 a is themaster which transmits data with the clock of its own, while the otherdata transmission devices 1 b to 1 f are slaves which operatesynchronized with a clock generated at the master.

In FIG. 5, operations at steps S31, S32, and S71 of the datatransmission devices 1 a to 1 f are identical to those of theabove-described steps S11, S12, and S51; therefore, a detailedexplanation thereof is omitted.

If, at the above-described step S32, the MPU 3 included in the masterdata transmission device 1 a has determined to shift to the zero-powermode, the MPU 3 uses an operation mode signal or the like to notify thetransmission/reception section 4 of its own device to shift to thezero-power mode, and the transmission/reception section 4 shifts to thezero-power mode (step S33). Then, the transmission/reception section 4stops output of the electric signal Mout being outputted from thetransmission section 6 to the transmission line 80 a (step S34). Notethat, at step S33, the transmission/reception section 4 may be caused toshift to the zero-power mode by the MPU 3 stopping supplying power tothe transmission/reception section 4. Further, at the above-describedstep S33, the MPU 3 may also notify the controller 2 of its own deviceto shift to the zero-power mode, so that the controller 2 shifts to thezero-power mode. In this case, the controller 2 included in the masterdata transmission device 1 a stops output of the digital data TX.

Through the processes of the above-described steps S32 to S34, themaster data transmission device 1 a completes the shift to thezero-power mode. This zero-power mode eliminates the need of theoperation of the controller 2 and transmission/reception section 4included in the data transmission device 1 a. Through the processes ofthe above-described steps S32 to S34, the controller 2 and thetransmission/reception section 4 are able to reduce power consumption tothe utmost by their own functions. Note that, however, after the processof the above-described step S33, the MPU 3 may perform power control onthe power supply section 8 to stop supplying power to the controller 2and the transmission/reception section 4. Moreover, if necessary, powersupply to the connected-device 10 a connected to the data transmissiondevice 1 a may also be stopped.

Meanwhile, the slave data transmission devices 1 b to 1 f eachdetermines, in the aforementioned normal operation, the presence orabsence of an input of the electric signal Min from the transmissionline 80 (step S72), and, if there is an input of the electric signalMin, continues the above-described step S71. Then, if the master datatransmission device 1 a performs the above-described step S34, therebystopping the output of the electric signal Mout being outputted to thetransmission line 80 a, the input of the electric signal Min to theslave data transmission device 1 b successively connected there to viathe transmission line 80 a ceases. By detecting that the input of theelectric signal Min has ceased, the transmission/reception section 4included in the master data transmission device 1 b shifts thetransmission/reception section 4 itself to the zero-power mode (stepS73), and stops output of the electric signal Mout (step S74). Then, thetransmission/reception section 4 in the data transmission device 1 bchanges the reception operation mode signal NST from High(l) to Low(0)and outputs it to the MPU 3 of its own device (step S75). Next, inresponse to the output of the reception operation mode signal NST beingchanged to Low(0), the MPU 3 of the data transmission device 1 b shiftsto the zero-power mode (step S76). Note that, in the process of stepS76, the MPU 3 of the data transmission device 1 b may notify thecontroller 2 of its own device to shift to the zero-power mode. In thiscase, the controller 2 included in the data transmission device 1 bshifts to the zero-power mode and stops output of the digital data TX.

Through the processes of the above-described steps S72 to S76, the slavedata transmission device 1 b completes the shift to the zero-power mode.Similarly to the master data transmission device 1 a, this zero-powermode eliminates the need of the operation of the controller 2 andtransmission/reception section 4 included in the slave data transmissiondevice 1 b. Through the processes of the above-described steps S72 toS76, the controller 2 and the transmission/reception section 4 are ableto reduce power consumption to the utmost by their own functions. Notethat, however, after the process of the above-described step S76, theMPU 3 may perform power control on the power supply section 8, therebystopping power supply to the controller 2 and the transmission/receptionsection 4. Moreover, if necessary, power supply to the connected-device10 b connected to the data transmission device 1 b may also be stopped.

The operation for shifting to the zero-power mode applied to the datatransmission device 1 b also applies to the other slave datatransmission devices 1 c to 1 f. That is, as a result of the input ofthe electric signal Min inputted from the transmission line 80 b havingbeen stopped, the data transmission device 1 c shifts to the zero-powermode; as a result of the input of the electric signal Min inputted fromthe transmission line 80 c having been stopped, the data transmissiondevice 1 d shifts to the zero-power mode; as a result of the input ofthe electric signal Min inputted from the transmission line 80 c havingbeen stopped, the data transmission device 1 e shifts to the zero-powermode; and, as a result of the input of the electric signal Min inputtedfrom the transmission line 80 e having been stopped, the datatransmission device 1 f shifts to the zero-power mode. The combinationof these operations causes all data transmission devices 1 a to 1 fconnected to the data transmission system to shift to the zero-powermode. In contrast to the operation of steps S52 to S58 shown in FIG. 3,the operation of steps S72 to S76 allows the transmission/receptionsection 4 itself to stop output of the electric signal Mout; therefore,the entire data transmission system shifts to the zero-power modequickly.

The foregoing description refers to the case where the data transmissiondevice 1 that starts the operation for shifting the data transmissionsystem from the normal operation mode to the zero-power mode is the datatransmission device 1 a which serves as the master in clocksynchronization. Note that, however, any of the other data transmissiondevices 1 b to in may start the operation for shifting to the zero-powermode. In this case, needless to say, one of the data transmissiondevices 1 b to in which starts the operation for shifting to thezero-power mode performs the operation of the master in FIG. 5, and theremaining data transmission devices perform the operations of slaves inFIG. 5, whereby all data transmission devices 1 a to 1 n are able toshift to the zero-power mode in a similar manner.

Next, with reference to FIG. 6, the operation of the data transmissionsystem for returning from the zero-power mode to the normal operationmode is described. The return operation of the data transmission devicedescribed below is also applicable to any system in which a plurality ofdata transmission devices 1 are connected so as to form a ringstructure. However, for making a specific explanation, the descriptionis made with reference to an exemplary case where six data transmissiondevices 1 a to 1 f are connected via transmission lines 80 a to 80 f soas to form a ring structure (see FIG. 1). Note that, as described above,when the data transmission system returns, the data transmission device1 a is the master which transmits data with the clock of its own, andthe other data transmission devices 1 b to 1 f are slaves which operatesynchronized with the clock generated at the master.

In FIG. 6, the operation at steps S41, S42, and S81 of the datatransmission devices 1 a to 1 f are identical to that of theabove-described steps S21, S22, and S61; therefore, a detaileddescription thereof is omitted.

If the MPU 3 included in the master data transmission device 1 a hasdetermined at the above-described step S42 to return to the normaloperation mode, the MPU 3 activates the controller 2 andtransmission/reception section 4 of its own device (step S43). Regardingthe activation at the above-described step S43, in the case where powersupply to be supplied from the power supply section 8 has been stoppedin order for the controller 2 and the transmission/reception section 4to shift to the zero-power mode, the MPU 3 controls the power supplysection 8 to resume power supply to the controller 2 and thetransmission/reception section 4. Further, in the case where thecontroller 2 and the transmission/reception section 4 have limited powerconsumption to zero by their own functions to shift to the zero-powermode, the MPU 3 performs an activation process by instructing each ofthem to become activated and reset.

Next, the transmission/reception section 4 included in the master datatransmission device 1 a, which has been activated by the above-describedstep S43, shifts to the normal operation mode. Then, thetransmission/reception section 4 performs an initialization operation ona physical layer, and, in the initialization operation, establishesclock synchronization with each data transmission device. Based on anoutput clock of a transmission PLL controlled by the clock controlsection 7 of its own device, the transmission/reception section 4transmits, to the transmission line 80 a, a lock signal LS forestablishing clock synchronization with another data transmissiondevice, as the electric signal Mout (step S44).

Meanwhile, the slave data transmission devices 1 b to 1 f are eachoperating in the zero-power mode, and the activity detection section 9of its own device monitors the presence or absence of input of theelectric signal Min from the transmission line 80 (step S82). If thereis no input of the electric signal Min, the above-described step S81continues. Then, if the master data transmission device 1 a performs theabove-described step S44, thereby outputting the lock signal LS as theelectric signal Mout to the transmission line 80 a, the input of theelectric signal Min to the slave data transmission device 1 bsuccessively connected thereto via the transmission line 80 a isstarted. If any input of the electric signal Min is detected, theactivity detection section 9 included in the slave data transmissiondevice 1 b outputs to the MPU 3 of its own device an activity detectionsignal for indicating that detection (step S83).

If, at step S83, there is any input of the activity detection signalfrom the activity detection section 9 included in the slave datatransmission device 1 b, the MPU 3 of its own device activates thecontroller 2 and transmission/reception section 4 of its own device(step S84). Regarding the activation at the above-described step S84, inthe case where power supply to be supplied from the power supply section8 has been stopped in order for the controller 2 and thetransmission/reception section 4 to shift to the zero-power mode, theMPU 3 controls the power supply section 8 to resume power supply to thecontroller 2 and the transmission/reception section 4. Further, in thecase where the controller 2 and the transmission/reception section 4have limited power consumption to zero by their own functions to shiftto the zero-power mode, the MPU 3 performs an activation process byinstructing each of them to become activated and reset.

Next, the transmission/reception section 4 included in the slave datatransmission device 1 b, which has been activated by the above-describedstep S84, shifts to the normal operation mode. Then, thetransmission/reception section 4 reproduces a clock with the clockreproduction section of its own device (step S85), and transmits a locksignal LS to the transmission line 80 b based on the clock outputted byits own reception PLL (step S86).

The operation for returning to the normal operation mode applied to thedata transmission device 1 b also applies to the other slave datatransmission devices 1 c to 1 f. That is, as a result of the activitydetection section 9 detecting an input of the electric signal Mininputted from the transmission line 80 b, the data transmission device 1c returns to the normal operation mode; as a result of the activitydetection section 9 detecting an input of the electric signal Mininputted from the transmission line 80 c, the data transmission device 1d returns to the normal operation mode; as a result of the activitydetection section 9 detecting an input of the electric signal Mininputted from the transmission line 80 d, the data transmission device 1e returns to the normal operation mode; and, as a result of the activitydetection section 9 detecting an input of the electric signal Mininputted from the transmission line 80 e, the data transmission device 1f returns to the normal operation mode. That is, as a result of thecombination of these return operations, the return to the zero-powermode is performed in regular order, starting with the master datatransmission device 1 a. Then, the transmission/reception section 4 ofthe data transmission device 1 f reproduces a clock with the clockreproduction section of its own device, and transmits a lock signal LSto the transmission line 80 f based on the clock outputted from thereception PLL.

After transmitting the lock signal LS to the data transmission device 1b at the above-described step S44, the master data transmission device 1a continues to be ready for a reception of the lock signal LStransmitted from the data transmission device 1 f via the transmissionline 80 f (step S45). Then, if the slave data transmission device 1 fperforms the above-described step S86, thereby outputting the locksignal LS as the electric signal Mout to the transmission line 80 f, thetransmission/reception section 4 included in the master datatransmission device 1 a receives the lock signal LS from thetransmission line 80 f and reproduces the clock with the clockreproduction section of its own device. Thus, clock synchronization ofthe entire data transmission system is established.

Thereafter, the data transmission devices 1 a to 1 f transmit andreceive a training signal for setting a criterion for data receptionbetween them, thereby setting a criterion for data determination in thenormal operation mode, and then start data transmission and reception(steps S46 and S87). Then, the process according to this flowchart iscompleted.

As described above, the data transmission system in which a plurality ofdata transmission devices are connected via the transmission lines so asto form a ring structure and in which the data transmission deviceselectrically communicate with one another in a single direction turnsoff the controller and transmission/reception section included in eachdata transmission device in the zero-power mode, in which power of mainhardware is turned off to suspend operation; therefore, powerconsumption is made extremely reduced in the zero-power mode. Inaddition, when the data transmission system returns from the zero-powermode to the normal operation mode, the master data transmission devicereturns to the normal operation mode upon agreement with a predeterminedcondition for return being obtained. In addition, the other slave datatransmission devices return in combination, as a result of detecting,with the activity detection section, an electric signal transmitted froma preceding data transmission device. Therefore, for example, even inthe case of a data transmission system where the MOST is used as thecommunications protocol to perform electrical communication, it ispossible to allow the entire system to return easily.

The activity detection section 9 provided in the data transmissiondevice 1 has been described as being disposed outside thetransmission/reception section 4 independently. Note that, however, itmay be disposed inside a transmission/reception section 4 composed of anLSI. In this case, if it is so arranged that only the activity detectionsection 9 disposed inside the LSI operates in the zero-power mode, thereturn to the normal operation mode is accomplished in a manner similarto that of the above-described flowcharts. In addition, the foregoingdescription refers to the case where the data transmission device 1 thatstarts the operation for returning the data transmission system from thezero-power mode to the normal operation mode is the data transmissiondevice 1 a which serves as the master in clock synchronization. However,any of the other data transmission devices 1 b to 1 n may start theoperation for returning to the normal operation mode. In other words,even in the case where any one of the slave data transmission devices 1b to 1 n first sends a lock signal LS, the activity detection section 9of each of the other data transmission devices is able to performactivation by detecting an electric signal Min.

INDUSTRIAL APPLICABILITY

A data transmission device, a data transmission system, and a datatransmission method according to the present invention realize shiftingin combination to the zero-power mode which considerably reduces powerconsumption of an entire device, and are usable as a device included ina system in which each device is connected via a transmission line so asto form a ring structure or the like to perform electricalcommunication, or as the system or the like.

1-23. (canceled)
 24. A data transmission device connected to a ring-typedata transmission network, which electrically communicates with anotherdevice via a transmission line in a unidirectional manner, the datatransmission device comprising: a processing section for processingreceived data and data to be transmitted based on a predeterminedcommunications protocol; a reception section for receiving an electricsignal sent from a preceding device and outputting data contained in theelectric signal to the processing section; a transmission section forconverting a result of a process by the processing section into anelectric signal and transmitting the electric signal to a successivedevice; a power supply section for supplying power to the processingsection, the reception section, and the transmission section; and acontrol section for controlling operation of the processing section, thereception section, and the transmission section in accordance with anoperation mode of its own device, wherein, the reception section detectscessation of the electric signal sent from the preceding device, if thereception section detects the cessation of the electric signal, thepower supply section stops supplying power to the processing section,the reception section, and the transmission section, in response toeither one of the cessation of the electric signal sent from thepreceding device being detected and power supply from the power supplysection being stopped, the reception section stops operating, and inresponse to either one of the reception section detecting the cessationof the electric signal and the power supply from the power supplysection being stopped, the transmission section stops operating andstops sending the electric signal to the successive device.
 25. The datatransmission device according to claim 24, wherein, if the cessation ofthe electric signal sent from the preceding device is detected, thereception section transmits, to the control section, a data cessationsignal for indicating the cessation, and based on the data cessationsignal transmitted from the reception section, the control section stopsoperation of the processing section.
 26. The data transmission deviceaccording to claim 24, wherein, if the cessation of the electric signalsent from the preceding device is detected, the reception sectiontransmits, to the control section, a data cessation signal forindicating the cessation, based on the data cessation signal transmittedfrom the reception section, the control section outputs a signal forstopping operation of the reception section and the transmissionsection, in response to the signal outputted from the control section inresponse to the detection, the reception section stops operating, and inresponse to the signal outputted from the control section in response tothe detection, the transmission section stops operating and stopssending the electric signal to the successive device.
 27. The datatransmission device according to claim 24, wherein, if the cessation ofthe electric signal sent from the preceding device is detected, thereception section transmits, to the control section, a data cessationsignal for indicating the cessation, and based on the data cessationsignal transmitted from the reception section, the control sectionperforms control of stopping the power supply section from supplyingpower to the processing section, the reception section, and thetransmission section.
 28. The data transmission device according toclaim 27, further comprising a signal monitoring section for detectingthe electric signal sent from the preceding device and transmitting, tothe control section, an electric-signal detection signal for indicatingthe detection, wherein, if suspended sending of the electric signal sentfrom the preceding device is resumed, the signal monitoring sectiondetects the electric signal sent from the preceding device, andtransmits, to the control section, the electric-signal detection signalfor indicating the detection, based on the electric-signal detectionsignal transmitted from the signal monitoring section, the controlsection performs control of allowing the power supply section to startsupplying power to the processing section, the reception section, andthe transmission section to start operation of the processing section,the reception section, and the transmission section, and by control ofthe control section, the transmission section starts operating andstarts sending the electric signal to the successive device.
 29. Thedata transmission device according to claim 28, wherein the electricsignal which the transmission section sends to the successive deviceafter starting operating by control of the control section is a locksignal for establishing clock synchronization.
 30. The data transmissiondevice according to claim 24, wherein the communications protocol usedby the processing section is defined by Media Oriented Systems Transport(MOST).
 31. A data transmission system including a plurality of datatransmission devices connected via a transmission line so as to form aring structure, in which the data transmission devices electricallycommunicate with one another in a unidirectional manner, the datatransmission devices each comprising: a processing section forprocessing received data and data to be transmitted based on apredetermined communications protocol; a reception section for receivingan electric signal sent from a preceding data transmission device andoutputting data contained in the electric signal to the processingsection; a transmission section for converting a result of a process bythe processing section into an electric signal and transmitting theelectric signal to a successive data transmission device; a power supplysection for supplying power to the processing section, the receptionsection, and the transmission section of its own device; and a controlsection for controlling operation of the processing section, thereception section, and the transmission section in accordance with anoperation mode of its own device, wherein, in at least one of the datatransmission devices, the control section stops operation of theprocessing section, the reception section, and the transmission sectionof its own device based on a predetermined condition for shift, and thetransmission section stops transmission of the electric signal, and inanother data transmission device, the reception section of its owndevice detects cessation of the electric signal sent from the precedingdata transmission device, if the reception section detects the cessationof the electric signal, the power supply section of its own device stopssupplying power to the processing section, the reception section, andthe transmission section, in response to either one of the cessation ofthe electric signal sent from the preceding data transmission devicebeing detected and power supply from the power supply section of its owndevice being stopped, the reception section of its own device stopsoperating, and in response to either one of the reception section of itsown device detecting the cessation of the electric signal and the powersupply from the power supply section of its own device being stopped,the transmission section of its own device stops operating and stopssending the electric signal to the successive data transmission device.32. The data transmission system according to claim 31, wherein, in theother data transmission device, if the cessation of the electric signalsent from the preceding data transmission device is detected, thereception section transmits, to the control section of its own device, adata cessation signal for indicating the cessation, and based on thedata cessation signal transmitted from the reception section of its owndevice, the control section stops operation of the processing section ofits own device.
 33. The data transmission system according to claim 31,wherein, in the other data transmission device, if the cessation of theelectric signal sent from the preceding data transmission device isdetected, the reception section transmits, to the control section of itsown device, a data cessation signal for indicating the cessation, basedon the data cessation signal transmitted from the reception section ofits own device, the control section outputs a signal for stoppingoperation of the reception section and the transmission section of itsown device, in response to the signal outputted from the control sectionof its own device in response to the detection, the reception sectionstops operating, and in response to the signal outputted from thecontrol section of its own device in response to the detection, thetransmission section stops operating and stops sending the electricsignal to the successive data transmission device.
 34. The datatransmission system according to claim 31, wherein, if the cessation ofthe electric signal sent from the preceding data transmission device isdetected, the reception section transmits, to the control section of itsown device, a data cessation signal for indicating the cessation, andbased on the data cessation signal transmitted from the receptionsection of its own device, the control section performs control ofstopping the power supply section of its own device from supplying powerto the processing section, the reception section, and the transmissionsection.
 35. The data transmission system according to claim 34,wherein, the data transmission devices each further comprise a signalmonitoring section for detecting the electric signal sent from thepreceding data transmission device and transmitting, to the controlsection, an electric-signal detection signal for indicating thedetection, in at least one of the data transmission devices, based on apredetermined return condition, the control section performs control ofallowing the power supply section to start supplying power to theprocessing section, the reception section, and the transmission sectionof its own device in stopped state to start operation of the processingsection, the reception section, and the transmission section, and thetransmission section resumes the transmission of the electric signal,and in another data transmission device, if suspended sending of theelectric signal sent from the preceding data transmission device isresumed, the signal monitoring section detects the electric signal sentfrom the preceding data transmission device, and transmits, to thecontrol section of its own device, the electric-signal detection signalfor indicating the detection; based on the electric-signal detectionsignal transmitted from the signal monitoring section, the controlsection performs control of allowing the power supply section to startsupplying power to the processing section, the reception section, andthe transmission section of its own device to start operation of theprocessing section, the reception section, and the transmission section;and the transmission section starts operating and starts sending theelectric signal to the successive data transmission device.
 36. The datatransmission system according to claim 35, wherein the electric signalwhich each transmission section sends to the successive datatransmission device after starting operating by control of the controlsection is a lock signal for establishing clock synchronization witheach other.
 37. The data transmission system according to claim 36,wherein the data transmission device which resumes the transmission ofthe electric signal based on the predetermined return condition is amaster, which performs data transmission with a clock held thereby andis connected to the data transmission system.
 38. The data transmissionsystem according to claim 31, wherein the communications protocol usedby the processing section is defined by Media Oriented Systems Transport(MOST).
 39. A data transmission method in which a plurality of nodes areconnected via a transmission line so as to form a ring structure andeach node electrically communicates with one another in a unidirectionalmanner, the method comprising: a processing step, performed by eachnode, of processing received data and data to be transmitted based on apredetermined communications protocol; a reception step, performed byeach node, of receiving an electric signal sent from a preceding nodeand sending data contained in the electric signal to the processingstep; a transmission step, performed by each node, of transmitting aresult of a process by the processing step to a successive node as anelectric signal; a power supply step of supplying power used foroperation in the processing step, the reception step, and thetransmission step; and a control step, performed by each node, ofcontrolling operation of the processing step, the reception step, andthe transmission step in accordance with an operation mode, wherein, inat least one of the nodes, the control step stops operation by theprocessing step, the reception step, and the transmission step of thenode based on a predetermined condition for shift, and the transmissionstep stops transmission of the electric signal, and in another node, thereception step of its own node detects cessation of the electric signalsent from the preceding node, if the reception step of its own nodedetects the cessation of the electric signal, the power supply step ofits own node stops supplying power used for operation of the processingstep, the reception step, and the transmission step of its own node, inresponse to either one of the cessation of the electric signal sent fromthe preceding node being detected and the power supply by the powersupply step of its own node being stopped, the reception step of its ownnode stops operation, and in response to either one of the receptionstep of its own node detecting the cessation of the electric signal andthe power supply step of its own node stopping supplying power, thetransmission step of its own node stops operation and stops sending theelectric signal to the successive node.
 40. The data transmission methodaccording to claim 39, wherein, in the other node, if the cessation ofthe electric signal sent from the preceding node is detected, thereception step sends, to the control step of its own node, anotification indicating the cessation, and based on the notificationsent by the reception step of its own node, the control step stopsoperation by the processing step of its own node.
 41. The datatransmission method according to claim 39, wherein, in the other node,if the cessation of the electric signal sent from the preceding node isdetected, the reception step sends, to the control step of its own node,a notification indicating the cessation, based on the notification sentby the reception step of its own node, the control step sends anotification for stopping operation by the reception step and thetransmission step of its own node, in response to the notification sentby the control step of its own node in response to the detection, thereception step stops operation, and in response to the notification sentby the control step of its own node in response to the detection, thetransmission step stops operation and stops sending the electric signalto the successive node.
 42. The data transmission method according toclaim 39, wherein, if the cessation of the electric signal sent from thepreceding node is detected, the reception step sends, to the controlstep of its own node, a notification indicating the cessation, and basedon the notification sent by the reception step of its own node, thecontrol step performs control of stopping the power supply step of itsown node from supplying power used for operation of the processing step,the reception step, and the transmission step.
 43. The data transmissionmethod according to claim 42, wherein, the nodes each further comprise asignal monitoring step of detecting the electric signal sent from thepreceding node and sending, to the control step, a notificationindicating the detection, in at least one of the nodes, based on apredetermined return condition, the control step performs control ofallowing the power supply step to start supplying power used foroperation of the processing step, the reception step, and thetransmission step of its own node in stopped state to start operation bythe processing step, the reception step, and the transmission step, andthe transmission step resumes the transmission of the electric signal,and in another node, if suspended sending of the electric signal sentfrom the preceding node is resumed, the signal monitoring step detectsthe electric signal sent from the preceding node, and sends, to thecontrol step of its own node, the notification indicating the detection;based on the notification indicating the detection sent by the signalmonitoring step, the control step performs control of allowing the powersupply step to start supplying power used for operation of theprocessing step, the reception step, and the transmission step of itsown node to start operation by the processing step, the reception step,and the transmission step; and operation by the transmission step isstarted to start the sending of the electric signal to the successivenode.
 44. The data transmission method according to claim 43, whereinthe electric signal which each transmission step sends to the successivenode after starting operation by control of the control step is a locksignal for establishing clock synchronization with each other.
 45. Thedata transmission method according to claim 44, wherein the node whichresumes the transmission of the electric signal based on thepredetermined return condition is a master, which performs datatransmission with a clock held thereby.
 46. The data transmission methodaccording to claim 39, wherein the communications protocol used by theprocessing step is defined by Media Oriented Systems Transport (MOST).